Lastly, I want to return to the point I made in the old post excerpted above. At this point Saruman was in a terrible position vis a vis Sauron. Saruman's only true hope of victory was to sieze the Ring first. There were two possibilities. Either Merry and Pippen had possessed it, in which case it was lost somewhere in the vicinity of Fangorn and he needed a free hand to search for it. Saruman might also reasonably conclude that on or about March 1 there was a risk that the Ring was in the hands of a champion of Rohan. If so - if Theoden or Eomer had the Ring - he had to kill them before they learned how to use it. The other alternative is that the Ring was still somewhere in the vicinity of the Anduin and headed for Minas Tirith. In that scenario he needed a free hand to hunt for the Ring bearer all the way on the far side of Rohan. I don't think Saruman would have felt that he could afford to leave Theoden and an army of Rohirrim at Helm's Deep. I think Saruman had no choice but to play for an immediate crushing victory. In the grand scheme things, Saruman's victory over Rohan would be meaningless if the Ring escaped him. He could never hope to defeat Sauron's forces without it. I think Saruman made a sound decision both tactically and strategically, but was undone by a few unlikely twists of fate.
He wasn't the only one. I don't think Sauron made bad choices. If Eowyn doesn't kill the Witch King, Gondor falls and Sauron wins. If Aragorn doesn't walk the Paths of the Dead and rally the Oathbreakers, Gondor falls and Sauron wins. If a hobbit and his servant don't walk alone into the most inhospitable and heavily guarded place in Middle Earth, all the way to Mt. Doom while evading capture, starvation, etc., Sauron wins. If Sam doesn't go into full Terminator mode and defeat Shelob, Sauron wins. Even with all of that, Frodo actually fails. At the final test, he can't find the strength of will to destroy the Ring.
Ultimately, it is only by the grace of Illuvatar that the light prevails and Sauron is defeated. That is not to denigrate the role of the heroes of the story. Their courage and sacrifice are in a sense the vehicles through which the grace of Illuvatar is made manifest. Without Frodo's very nearly indomitable will, he does not make it to the precipice for his encounter with Gollum. Without Frodo's pity, Gollum does not survive to the encounter. But for Faramir's wisdom, Frodo does not leave Ithilien with the Ring. But for actions of Aragorn and Eowyn, Gondor is defeated and Sauron is not distracted at the critical moment.
All of which is a very long winded way of saying that I disagree that Saruman made a foolish tactical decision to assault Helm's Deep.